docs: def _get_mechanism(self, rup, C):
          """
          Compute the fourth term of the equation described on p. 199:
  
          ``f1 * Fn + f2 * Fr``
          """
          Fn, Fr = self._get_fault_type_dummy_variables(rup)
          return (C['f1'] * Fn) + (C['f2'] * Fr)